Extraction of petrochemicals from subterranean formations is an important global industry. However, in North America and many parts of the world, petrochemicals are found in heavy, viscous forms such as bitumen, which are extremely difficult to extract. The bitumen-saturated oilsands reservoirs of Canada, Venezuela, California, China and other parts of the world are just some examples of such subterranean formations. In these formations, it is not possible to simply drill wells and pump out the oil. Instead, the reservoirs are heated or otherwise stimulated to reduce viscosity and promote extraction.
The two most common and commercially-proven methods of stimulating oilsands reservoirs are (a) cyclic steam stimulation (CSS) and (b) steam assisted gravity drainage (SAGD). In both cases, steam is injected into the reservoir, to heat up the bitumen. Some variations of these processes may involve injecting solvent to aid the viscosity reduction or use electrical heating to replace the role of steam.
In general, the initial injectivity into the reservoir, i.e., how much volume of the stimulant can be injected per unit of time without fracturing the formation, is relatively small. Stimulation of the reservoir is desired to provide channels via which the stimulant can travel to access and contact the reservoir. These channels not only increase injectivity, but also increase the contact area of stimulant within the reservoir.
In SAGD processes, before the production can start, communication between the SAGD well pair must be established so that the bitumen can flow down from an upper injection well to a lower production well. Conventionally, steam is circulated through the two wells independently until the inter-well area is heated and the bitumen viscosity is reduced significantly so that it can flow to the production well and material communication is established. This process normally takes up to 6 months to complete. More time may be needed in some situations such as when the well trajectory drilled deviates from the ideal pattern of vertically-aligned with 5 m apart.
The art of hydraulic fracturing as a stimulation method for hydrocarbon resource recovery has been practiced for a long time. In general, this method injects liquid at high pressures into a well drilled through the target formation to be stimulated. The high pressure initiates a fracture from the injection well and propagates a sufficient distance into the formation. Then, the fracture is filled with proppants that are injected from the surface after the fracture is formed. The similar method is applied in vertical and horizontal wells and wells of any inclinations. However, the existing art of hydraulic fracturing is subject to two major limitations:                Hydraulic fracturing has no proactive control of the orientation of the fracture formed. The fracture follows the plane perpendicular to the least resistance, i.e., perpendicular to the original in-situ minimum stress, Smin0. If a horizontal well is drilled in the direction of minimum stress Smin0 or substantially inclined towards it, a fracture being formed via conventional hydraulic fracturing may be perpendicular or substantially perpendicular to the horizontal well. Such fractures do not contribute to the uniform communication between the SAGD wells along their length.        Selective placement of hydraulically-driven fractures in the plane perpendicular to the original in-situ maximum stress, Smax0, has been practiced in the past. However these typically require a sacrificial well, which adds cost to drilling and completion of the SAGD well pair. Moreover, the sacrificial fracture formed in the process will complicate steam conformance and thus makes inter-well communication difficult between the SAGD well pair.        
As a norm, many local inhomogeneities exist in the in-situ conditions and in the operating well conditions along the SAGD well length. Given these inhomogeneities, whether manually-induced or pre-existing along the well length, it is highly unlikely that the fracture formed in conventional hydraulic fracturing can propagate uniformly or continuously along the length of the horizontal wells, which can span over 800 m, unless the horizontal well length is segmented to be treated at different times.
The goal of the conventional hydraulic fracturing stimulation is to form a fracture, more specifically an open fracture which represents a geomechanically thin linear or planar defect. It is often tensile in mechanical nature and modeled by two parallel plates with an open aperture between. Pressure or fluid conductivity through such an open fracture is often very high. If an open fracture is formed between the SAGD wells locally along the well length, essentially no pressure drop occurs along the fracture. Thus, it can lead to continuous propagation along its linear or planar path and it does not promote lateral propagation of the fracture, i.e., uniform propagation along the SAGD well length.
There has been some work done in controlling the orientation of fractures. For example, U.S. Pat. No. 3,613,785 by Closmann (1971) teaches creating a horizontal fracture from a first well by vertically fracturing the formation from a second well and then injecting hot fluid to heat the formation. Heating via the vertical fracture alters the original in-situ stress so that the vertical stresses become smaller than horizontal stresses, thus favouring a horizontal fracture being formed. This method requires a first sacrificial vertical fracture be formed and uses costly steam to heat the formation.
U.S. Pat. No. 3,709,295 by Braunlich and Bishop (1971) controlled the direction of hydraulic fractures by employing at least three wells and a natural fracture system. This method is only feasible in formations already having existing fractures.
U.S. Pat. No. 4,005,750 by Shuck (1975) teaches creating an oriented fracture in the direction of the minimum in-situ stress from a first well by first hydraulically fracturing another well to condition the formation. Again, additional wells and sacrificial fractures are required before the targeted fracture can be formed.
Canadian patent CA 1,323,561 by Kry (1985) teaches creating a horizontal fracture from a center well after cyclically steam-stimulating at least one peripheral well. At the peripheral well a vertical fracture is created. CSS operations coupled with fracturing at the peripheral well conditions the stress field so that a horizontal fracture can be formed. To create the horizontal fracture, a high-viscosity fluid is proposed to inject into the center well to limit the fluid from leaking into the formation.
Canadian patent CA 1,235,652 by Harding et al. (1988) teaches first vertically-fracturing the formation from peripheral wells to alter or condition the in-situ stress regime in the center region of the peripheral wells. The formation is then fractured through a central well to create and extend a horizontal fracture.
All of the above documents require either the existence of a natural fracture in the formation already or the formation of sacrificial fractures before a targeted fracture can be induced. Furthermore, many of these documents have specific requirements for heated or highly viscous injection fluids to condition the formation or to induce the targeted fracture.